This invention relates generally to the chemical modification of surfaces so as to provide a pattern thereon, and more particularly relates to a process for preparing a patterned continuous polymeric brush on a substrate surface. The invention has utility in the fields of microelectronic circuitry and semiconductor manufacturing, solid phase chemical synthesis, and biotechnology, e.g., in preparing biosensors, high-density assay plates, and the like.
A well-known method for manufacturing electronic devices such as integrated circuits and the like involves photolithography, wherein a radiation-sensitive lithographic photoresist composition is applied through spin casting as a thin coating on the exposed surface of a substrate. The coating is subsequently xe2x80x9cbakedxe2x80x9d to remove the casting solvent. The photoresist film is then exposed to radiation in an imagewise fashion, and the image is developed, typically by immersion in a developer solvent. Because of the difference in solubility between the exposed and unexposed photoresist in the developer solvent, either the exposed or unexposed portion of the photoresist is washed from the surface to produce a predetermined pattern of resist on the surface. With a so-called xe2x80x9cpositivexe2x80x9d resist, it is the exposed areas that are removed, while with a xe2x80x9cnegativexe2x80x9d resist, it is the unexposed areas that are removed. Advantages in photolithographically patterning spin-cast films include high resolution of features and reproducibility.
However, attempts to perform chemistry on patterned spin-cast films, or to employ these films in solvating environments, often fail because the films dissolve or swell in typical reaction media. Thus, there exists a need in the art for a reproducible method of preparing a substrate surface according to a predetermined pattern wherein the pattern is stable to a wide range of different environments and processing conditions, e.g., wherein any dissolution or swelling of the pattern composition is negligible, and wherein chemistry can be performed on the surface pattern with no dissolution, degradation or removal thereof The method would ideally result in patterns having discrete surface properties such as hydrophilic and hydrophobic portions with resolution in at least the micron or submicron domain.
Polymer brushes, in contrast to patterned spin-cast films, are stable with respect to a range of environmental and processing conditions. As with preparation of self-assembled monolayers (SAMs), polymer brushes are typically formed by first depositing initiating groups on a substrate surface that covalently bind thereto. Then, macromolecular chains are grown from the initiating groups using monomers that are typically similar to those traditionally used in microlithography, e.g., t-butyl acrylate. The covalent bonding of the macromolecular chains to the substrate surface opens up a number of possibilities that are not available with traditional spin-cast films. These advantages permit the use of these films in technological applications that include specialty photoresists, sensors and microfluidic networks.
A number of different approaches to synthesis of patterned polymer brushes have been described. For example, Ruhe et al. have reported the patterning of surface bound initiators by either photoablation or photoinitiation, followed by polymerization to give discrete areas of polymer brushes (Prucker et al. (1998) Macromolecules 31 592; Prucker et al. (1998) Macromolecules 31:602; Ruhe et al. (1997) J. Macromol. Symp. 126:215; Biesalski et al. (1999) Macromolecules 32:2309), while Clark et al. have detailed the growth of patterned polymer films using layer by layer techniques (Clark et al. (1998) Adv. Mater. 10:1515). In addition, a number of groups have also reported the elaboration of microcontact printed thiol monolayers to provide patterned polymer brushes; see, for example, Yan et al. (1 999) Langmuir 15:1208, Lackowski et al. (1999) J. Am. Chem. Soc. 121:1419, and Husemann et al. (1999) Angew Chem. Int. Ed. Engl. 38:647. Yan et al. describes patterned thin films of poly(ethylene imine) on reactive SAMs, while Lackowski et al. has employed microcontact printing to prepare a patterned thiol monolayer on Au containing discrete areas of functional groups that are transformed into patterned hyperbranched poly(acrylic acid) films using a stepwise growth strategy as described by Zhao et al. (1999) J. Am. Chem. Soc. 121:923. The structures produced by these two strategies are similar in that polymer brushes are not continuous over the substrate surface, with the well-defined areas of polymer brushes being separated by substrate surface/monolayer. However, none of these approaches addresses the alternative and potentially more useful and versatile structure in which a continuous polymer brush is patterned into chemically distinct regions.
This invention is addressed to the aforementioned need in the art, and provides a method to form polymer brushes that are patterned so as to contain discrete and well-defined regions that are chemically distinct and have different surface properties, e.g., discrete regions of hydrophilic and hydrophobic polymer brushes. Such polymer brushes, having domains with different surface properties, enable control of surface chemistry and can be used in a variety of ways to take advantage of differential surface interaction with external agents, e.g., wet and dry etchants and the like. The method is readily adaptable for use with current semiconductor processing technologies, and provides an extremely stable, high resolution, versatile product.
Accordingly, it is a primary object of the present invention to address the above-mentioned need in the art by providing a method for forming a continuous polymeric brush that is patterned into two or more chemically distinct regions, wherein the method generally involves:
(a) providing a substrate having a surface to which molecular moieties can covalently bind;
(b) contacting the surface with a polymerization composition under conditions effective to provide a continuous polymer brush, wherein the polymerization composition is such that the continuous polymer brush contains labile groups selected from the group consisting of acid-labile, base-labile, and photolabile groups; and
(c) applying a cleavage agent selected from the group consisting of acid, base, and radiation to the continuous polymer brush in a predetermined pattern under conditions that promote reaction between the labile groups and the cleavage agent.
In a preferred embodiment, the surface is initially modified by reaction with a derivatizing composition comprised of a derivatizing moiety having a reactive site for reaction with the surface and a functional group, such that surface-bound functional groups result that serve as polymerization sites. A surface-initiated polymerization process is then conducted, e.g., a living free radical polymerization process, using reactive monomers having acid, base or photolabile groups. Application of a cleavage agent (i.e., an appropriate chemical reagent and/or radiation) to the continuous polymer brush so provided is carried out in a manner that results in removal of the labile groups in at least one first region of the polymer brush and but not in at least one second region of the polymer brush.
In one embodiment, acid-labile groups are used in conjunction with an acid cleavage agent wherein the application of the cleavage agent to the continuous polymer brush is carried out by depositing a film containing a quantity of a radiation-sensitive acid generator (also termed a xe2x80x9cphotoacid generator or xe2x80x9cPAGxe2x80x9d) onto the continuous polymer brush and subsequently exposing selected regions of the film to radiation, whereby the exposed region then release the photogenerated acid into the polymer brush and react therewith. In another acid-labile/acid cleavage agent embodiment, the polymer brush is impregnated with a photoacid generator and the impregnated brush is imagewise exposed to radiation such that selected regions undergo reaction with the photogenerated acid. Alternatively, base-labile groups and a basic cleavage agent can be used in an analogous manner.
In yet another embodiment, photolabile groups are utilized in conjunction with radiation. The surface-initiated polymerization process is conducted using reactive monomers having photolabile groups attached thereto. The resulting photolabile containing polymer brush is then imagewise exposed to radiation resulting in removal of the photolabile groups in at least one first region of the polymer brush and not in at least one second region of the polymer brush.